Metals can be extracted from a number of primary and secondary sources. Primary sources include ores and naturally occurring solutions and mixtures comprising metal ions. Secondary sources include wastes, spent catalysts, process residues and mine tailings. In view of the scarcity of some metals and the cost and difficulties involved in the extraction of metals from primary sources, recovery processes for extracting metals from secondary sources are key.
Of particular interest are precious metals, and specifically the platinum-group-metals which are expensive due to their low natural abundance and the complex processes that are required for their extraction and refining from primary sources. Whilst the platinum-group metals are found as naturally occurring ores, for example sperrylite (platinum arsenide), these metals may also be obtained by recycling platinum-group-metal bearing wastes. The platinum-group-metals have outstanding catalytic properties and suitable waste is often catalytic waste such as crushed autocatalyst ceramic monolith, catalytic soot filters or heterogeneous process catalyst on a variety of different substrates.
Known methods of obtaining precious metals include cupellation, hydrometallurgical leaching and thermal decomposition and/or recovery processes. Thermal systems using plasma arcs are known, and have been applied to platinum-group metal recovery. High-temperature plasma arcs are heat sources of intense power and versatility, combining the heat of a plasma arc with the stability and control of a gas flame.
In a submerged arc furnace (SAF) a graphite electrode that is in direct contact with the melt is used as the means of providing electrical power. The graphite electrode is very large due to the high currents employed and, therefore, requires heavy lifting and handling equipment. This results in exaggerated graphite consumption of the order of 20 kg MWh−1. Such furnaces operate on a batch process and the feedstock is formulated to deliver the correct slag resistivity/chemistry as energy is provided to the process through resistive heating.
U.S. Pat. No. 4,685,963 discloses a process for the extraction of platinum-group-metals from various feedstocks where a plasma ‘flame’ at between 5,000° C. and 10,000° C. is used to superheat the upper slag layer to accelerate association within the collector material due to improved fluid flow patterns. The ‘superheated zone’ produces extreme localised heating and boiling, and churns and stirs the molten furnace contents. Reference is made to small ‘particles’ of platinum-group metals and collector material suspended in the slag and to the role of fluidity in their aggregation. These individual phases agglomerate discretely and migrate to become associated with each other at the base of the vessel.
There is, therefore, a desire for a process and/or apparatus that will improve the recovery of precious-metals, or at least mitigate some, or all, of the problems associated with the prior art, or provide a useful alternative.